All metioned would be of value though not necessraily as independents. There could be variations in unit types in the human armies as Viking or perhaps the addition of this type of unit could only be allowed when specific lands are conquered. The same could apply to dragons once a lair is overtaken.

i voted on dragons, but why not have insectoid races(mix of bees, beetles, ants, and as buildings: hives, tunnels, ant hills.
or a snake race a bit like the naga from warcraft.
I dont like the idea of having vikings.
End